Wavelength Division Multiplexing (WDM) is a basic technology of networking optical signals. It is a technique by which a single fiber is used to carry many separate and independent optical channels. Each channel within the optical wavelength division multiplexed (OWDM) network is assigned a separate optical wavelength at which it is transmitted through the network. In a “sparse” OWDM system, the optical wavelengths are (relatively) widely separated. For example, two optical wavelengths of 1300 nm and 1550 nm may be used in a sparse system. Such a system has an advantage of being easily implemented, but a major disadvantage is the limited number of optical channels that can be carried. In a “dense” OWDM system, the optical wavelengths are closely spaced. In a typical dense optical wavelength division multiplexed (DOWDM) system, the channel spacing may be as small as 1 nm or less. DOWDM systems provide substantially more channels than a sparse system, but are also more complex and difficult to implement.
OWDM technology provides the ability, in a given network, to allocate different services (or area of coverage) to different optical wavelengths for direct addressing. One example is in hybrid fiber coax WDM technology, where each service (broadcast video, pay per view, etc.) or different housing communities are routed by a designated wavelength in a Passive Optical Network (PON) architecture. Allocation of different services to different wavelengths simplifies the distribution of these services via optical networks, since the distribution hardware used in the networks does not need to know the type of service carried at each wavelength.
OWDM networks provide the capability to transmit large amounts of data between locations, but they have a fundamental limitation. OWDM networks require the use of optical fiber to move data from one point to another. Hence, OWDM networks may not serve areas where installation and maintenance of optical fiber is difficult and expensive. These areas may comprise rural areas where there are few users and these users are separated by significant distances, making the provision of fiber uneconomical. These areas may also include densely populated urban areas where the costs of interfering with the infrastructure and providing OWDM fiber to multiple locations may be prohibitively expensive.
OWDM networks may still be deployed in these areas, but they will generally be deployed in a relatively small area, servicing users who are closely located. Links to users on other networks may be accomplished by tying the networks together through the use of OWDM fiber or through the use of lower performing non-WDM data links. Coupling the separate OWDM networks through the use of OWDM fiber has the difficulty and expense factors discussed above. Non-WDM data links (such as radio frequency links or telephone lines) can be provided at a lower cost, but the networking capabilities inherent in a WDM network are lost.
A system for linking separate OWDM networks is described and claimed in the copending patent application entitled “Wireless Wavelength Division Multiplexed System,” Ser. No. 09/897,747, filed on Jun. 29, 2001. The present patent application discloses the conversion of data transported on each optical wavelength in an OWDM system to a corresponding microwave or millimeter-wave frequency in a one-to-one correspondence. One technique which may be used for converting the several modulated optical carriers in an OWDM network to modulated radio frequency carriers transporting the same information as in the OWDM network, is to first de-multiplex the OWDM optical carriers and then detect the information in each channel using separate photodetectors. The photodetectors essentially convert the optical signals to electrical signals. The data in the individual electrical signals then modulates separately generated radio frequency carriers for wireless transmission with a one-to-one correspondence to the optical channels in the OWDM network.
However, the generation of low phase noise signals at microwave or millimeter wave frequencies using standard electrical frequency synthesizers may be a costly process, since several multiplication stages of a high quality, low frequency signal to the microwave or millimeter wave frequencies are required. Also, frequency synthesizers capable of this task can be quite bulky. Therefore, as the number of channels in the OWDM system, and hence in the converted wireless link, increases, the use of standard electrical frequency synthesizers to provide the required conversion can become detrimental in terms of cost and size.
Therefore, there exists a need in the art for apparatus and methods that provide for the conversion of the optical channels in OWDM system to radio frequency channels in a less costly and bulky fashion.